As shown in FIG. 1, a rotary stencil printing apparatus is provided with a cylindrical printing drum 1 which rotates around its central axis, and the drum 1 has an ink passing peripheral wall composed of a material having many pores or a mesh material. This drum 1 can generally be made by a method of rolling a plate material such as a punching metal and an expanded metal into a cylindrical form and holding both ends by flanges, a method of fixing a part of a mesh material such as a screen to a flange to hold it in a cylindrical form, or a method of fitting flanges at both ends of a cylindrical shaped article, and the basic strength of the peripheral wall of the drum is determined by the materials constituting the drum. If necessary, one or more screen layers of micro-reticulate structure (not shown) can be wound around the peripheral surface of the drum 1.
Inside the drum 1, is provided an ink supplying means comprising a squeegee roller 4 which contacts with the inner peripheral surface of the drum, a doctor roller 5 which is disposed adjacent the squeegee roller, and others. Outside the drum 1, is disposed an opposing roller 6 at the position opposite to the squeegee roller 4, and the opposing roller 6 functions as a means of carrying a printing paper 10 in cooperation with the drum 1 with nipping the printing paper therebetween at the time of printing. According to FIG. 1, the opposing roller 6 is provided so that it can contact with and leave from the drum 1, and has a function to press the printing paper 10 against the outer peripheral surface of the drum 1 at the time of printing. Alternatively, the opposing roller 6 may have a diameter similar to that of the drum 1, and the squeegee roller 4 may have a function to press the peripheral wall of the drum 1 outwardly to deform the peripheral wall in such a manner that the peripheral wall can contact with and leave from the opposing roller 6, and the printing paper 10 may be carried with being nipped between the deformed drum and the opposing roller 6.
At the time of printing, one end of perforated stencil sheet 8 is clamped by a clamping means 2 provided at the outer peripheral surface of the drum 1 and the stencil sheet 8 is wound around the outer peripheral surface of the drum 1. When the drum is rotated, ink is supplied to the peripheral surface from inside of the drum 1 by the squeegee roller 4 and excess ink forms a reservoir of ink 7 between the squeegee roller 4 and the doctor roller 5. In this case, the printing paper 10 is carried with being nipped between the opposing roller 6 and the outer peripheral surface of the drum 1, whereby the ink is transferred to the printing paper through the stencil sheet 8 to perform printing. Under usual printing conditions, the drum 1 undergoes some deformation by pressing force at the nip portion at which the opposing roller 6 and the drum 1 contact with each other, but the apparatus must be designed so that other portions of the drum 1 are not deformed at the time of rotating in order to prevent slippage of the stencil sheet or creasing of the printing paper at the time of carrying the paper.
Although the peripheral wall of the drum can keep cylindrical shape under usual printing conditions, it sometimes undergoes deformation under some using conditions such as low temperature and high-speed printing. The similar phenomenon is also seen in an apparatus provided with a large drum designed so as to apply to the use of large-sized printing papers. As shown in FIG. 2, this deformation appears as a phenomenon that at the time of rotation of the drum 1 the periphery wall of the drum 1 deforms in the direction towards the squeegee roller 4 at the position 9 which is slightly downstream in rotation direction from the contact portion of the squeegee roller 4 and the drum 1, and this causes unevenness in the resulting printed images.